Whatsapp theme suddenly turned dark

I was chatting on whatsapp, then I didn't look at my phone for 5 seconds, and when I looked back I saw that it turned dark theme with a wall paper Why? And now I can not get it back to previous theme, even if I select default theme or light theme etc. How to return to previous theme? I was using default. I had not changed any setting for theme or Wallpaper I use latest version of android (I guess) , and my phone is Samsung Galaxy S9